The kinetics of the reaction of N,N-dimethyl-2-phenylaziridinium ion with bovine erythrocyte acetylcholinesterase

Abstract

The kinetics of the hydrolysis of N,N-dimethyl-2-phenylaziridinium ion (DPA) have been studied over the pH range 5.5–8.0 as have the kinetics of the interaction of DPA with bovine erythrocyte acetyl-cholinesterase. The enzyme is initially inhibited reversibly and subsequently irreversibly towards acetylcholine hydrolysis. The hydrolysis of DPA was found to be pH independent over the range studied while the reversible noncompetitive inhibition increased with increasing pH, the data suggesting the requirement for a basic group on the enzyme with a pKa of about 6.5.Between pH values of 6.0 and 8.0 the kinetics of the irreversible inhibition are consistent with either of two kinetically indistinguishable mechanisms, one involving transformation of the initial reversible complex and the other an independent attack on the uncomplexed enzyme. The first mechanism gives rise to a first-order rate constant which is comparable with that for the hydrolysis of DPA but which increases with decreasing pH; an acidic group on the enzyme with pKa between 6.0 and 7.0 may be involved. The second-order rate constant arising from the second treatment goes through a maximum at pH 7.3. At pH 5.5 the kinetics are not consistent with either mechanism.